Add Differential parse cache to the GC daemon

Summary:
Add the differential parse cache to the GC. This is the largest object in the
system by a wide margin, I think.

This table is potentially gigantic which is why the script truncates it before
doing a schema change.

Test Plan: Ran the GC daemon, it cleaned up some parse caches.
Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran
Commenters: tuomaspelkonen
CC: aran, jungejason, tuomaspelkonen, epriestley
Differential Revision: 620